The invention relates to a honeycomb body, particularly for an apparatus for catalytic conversion of exhaust gases in an exhaust system, in particular for an exhaust system of an internal combustion engine, preferably an Otto engine. The honeycomb body, through which an exhaust gas can flow, is disposed in a jacket tube and has flow channels that are formed between adjacent wound or stacked metal layers, at least some of which are structured. The honeycomb body is brazed to the jacket tube over only part of its axial length.
One such honeycomb body is known from German Published, Non-Prosecuted Patent Application DE 29 24 592 A1. In order to connect the honeycomb body to the jacket tube, German Published, Non-Prosecuted Patent Application DE 29 24 592 A1 proposes, among other things, that the end surfaces of the honeycomb body disposed in the jacket tube be dipped in an immersion bath for a binder, thus producing wetted zones. A brazing material coating is produced by applying brazing material from a reservoir of powdered brazing material through a sieve onto the wetted end surface. The thus-coated honeycomb body is brazed to the jacket tube in a vacuum or in a protective gas atmosphere.
It is also known from German Published, Non-Prosecuted Patent Application DE 29 24 592 A1 to dip the end surfaces of the honeycomb body disposed in the jacket tube in molten brazing material.
A honeycomb body which is disposed in a jacket tube and through which an exhaust gas can flow is known from German Published, Non-Prosecuted Patent Application DE 43 06 052 A1, corresponding to U.S. Pat. No. 5,419,878. The honeycomb body is formed of a smooth and a corrugated metal layer, which are spirally rolled while touching each other so that flow channels are formed between the metal layers. The smooth metal layer is thereby located on the outside. In order to provide a reliable brazing connection to the jacket tube, the smooth metal layer is constructed with several openings at equally distanced locations along the outer circumference.
A method for brazing carrier bodies of catalytic converters is known from Published International Patent Application WO 93/12908. The application of a brazing material to the metal layers of the honeycomb body is carried out by first dispersing a powdered brazing material of suitable particle size in a liquid mixture of binder material and liquid, and then flooding the flow channels of the honeycomb body. Excess mixture is removed from the flow channels after the flooding.
Both German Published, Non-Prosecuted Patent Application DE 29 24 592 A1 and Published International Patent Application WO 93/12908 use methods for producing a carrier body in which the outer region of the honeycomb body is brazed to the jacket tube. That produces a rigid connection between the honeycomb body and the jacket tube.
It is already known from Published International Patent Application WO 94/06594 to braze a honeycomb body to a jacket tube only in an axial subregion, by providing the jacket tube with brazing material from the inside in the subregion to be brazed and then thrusting the honeycomb body into the jacket tube.
In a further method for applying brazing material to a honeycomb body, which is known from Published International Patent Application WO 93/25339, corresponding to U.S. Pat. No. 5,431,330, brazing material is introduced into a honeycomb body from the end surfaces.
Most of the known brazing material application methods, and especially those in which brazing material is applied from the direction of the end surfaces, produce results in which not only are the metal layers of the honeycomb body brazed to one another, but there are also connections between the honeycomb body and the jacket tube. That is particularly true in honeycomb bodies built up from a plurality of metal layers extending approximately spirally or in an involute manner, but it is also true of spirally wound honeycomb bodies with a corrugated outer layer. The desired binding of the honeycomb body to the jacket tube only in a certain axial subregion cannot be achieved without problems in conjunction with such brazing material application methods. Often, additional connections between the honeycomb body and the jacket tube are created, especially at the end surfaces.
Since the honeycomb body and the jacket tube behave differently in terms of thermal expansion, a continuous rigid connection between the honeycomb body and the jacket tube is undesirable. Such a connection can allow thermal strains that can damage the honeycomb body to occur in the honeycomb body.
It is accordingly an object of the invention to provide a honeycomb body bonded only partially to a jacket tube, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which improves the known apparatus for catalytic conversion of exhaust gases in such a way that brazing of the honeycomb body to the jacket tube in certain axial regions is avoided, regardless of the brazing material application technique.
With the foregoing and other objects in view there is provided, in accordance with the invention, a honeycomb body disposed in a jacket tube and having an axial length and end surfaces; the honeycomb body being brazed to the jacket tube over a part of the axial length; the honeycomb body having mutually adjacent, at least partially structured wound or stacked metal layers and flow channels formed between the mutually adjacent metal layers for conducting an exhaust gas flow; and at least one of the layers having at least one smooth portion at least partially surrounding the honeycomb body and extending from at least one of the end surfaces over a part of the axial length.
The honeycomb body according to the invention, which is preferably suitable for an apparatus for catalytic conversion of exhaust gases in an exhaust system, is distinguished in that at least one layer of the honeycomb body has at least one smooth portion at least partially surrounding the honeycomb body and extending from at least one end surface over a part of the axial length of the honeycomb body. The smooth portion forms an outer layer on the circumference of the honeycomb body that rests on the jacket tube. Since the smooth portion rests on the jacket tube, no brazing material (or only slight amounts of brazing material) can travel between the smooth portion and the jacket tube, regardless of the brazing material application method employed. The application of the brazing material can be carried out in the manner known from the prior art. Joining of the honeycomb body to the jacket tube is carried out over a part of the axial length of the jacket tube, and the smooth portion extends axially of the honeycomb body only as far as the connection region between the honeycomb body and the jacket tube.
As a result, thermal expandability in the axial direction of the honeycomb body is attained, since the honeycomb body now is connected with the jacket tube only within one or more connection regions. No undesired brazing of the honeycomb body to the jacket tube takes place over the remainder of the honeycomb body and the jacket tube.
The smooth portion may be part of a smooth layer that is a component of the honeycomb body.
In accordance with another feature of the invention, one layer of the honeycomb body has a first smooth portion and a second smooth portion, each extending from the end surfaces over a part of the axial length of the honeycomb body, at most as far as a connection region between the honeycomb body and the jacket tube, and at least partially surrounding the honeycomb body. As a result of this provision, brazing material can be applied from one or both of the end surfaces of the honeycomb body, without creating a connection between the outer layer of the honeycomb body and the jacket tube. Another advantage of this embodiment is that in the manufacture of such a honeycomb body, even when brazing material is applied to only one side, it does not matter from which end surface the brazing material is introduced into a honeycomb body. This simplifies the manipulation and manufacture of such a honeycomb body.
In accordance with a further feature of the invention, the honeycomb body includes a stack of layers at least some of which are structured, the ends of the stack are each entwined in opposite directions about at least two fixed points, and two metal layers each have at least one smooth portion extending from at least one end surface over a part of the axial length of the honeycomb body and surrounding the honeycomb body. The layers can each form the topmost or bottommost layer of the stack. It is also possible for such layers to be disposed inside the stack. An embodiment is preferred in which the smooth portion or portions are constructed on a metal layer which is disposed in the vicinity of the middle of the stack. This has the advantage of attaining the desired success in the case of an S-shaped entwined honeycomb body with a single layer.
In accordance with an added feature of the invention, the smooth portion partially overlaps in the circumferential direction. This assures that the outer layer of the honeycomb body formed by the smooth portion is complete, so that a brazing agent cannot enter between the smooth portion and the jacket tube.
In accordance with a concomitant feature of the invention, the at least one smooth portion includes at least two smooth portions together entirely wrapping around the honeycomb body in circumferential direction.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a honeycomb body bonded only partially to a jacket tube, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.